1. Field of the Invention
The present invention relates to a power semiconductor apparatus for controlling a power and, more particularly, to a composite power semiconductor apparatus having a protection function together with an over-current sensing function and which incorporates an active power element and control elements having sensing functions of, e.g., a temperature, a voltage, and a current on a single chip semiconductor substrate.
2. Description of the Related Art
In a power semiconductor apparatus for controlling a power, when a fault such as short-circuiting occurs in a load controlled by the semiconductor apparatus, an overcurrent is supplied to this semiconductor apparatus. When such an overcurrent is supplied to the semiconductor apparatus for controlling a power, this semiconductor apparatus, a power source, or other peripheral devices may often receive fatal damage.
Such a power semiconductor apparatus, therefore, need be arranged such that a protection function is set to limit an amount of current supplied to the semiconductor apparatus when an excessive load current flows, thus protecting the semiconductor apparatus and its peripheral devices from breakage.
As a means for protecting such a semiconductor apparatus from an overcurrent, in general, e.g., an overcurrent protection circuit is externally connected to the semiconductor apparatus. More specifically, the overcurrent protection circuit arranged separate from the semiconductor apparatus senses an amount of current supplied to the semiconductor apparatus. When this protection circuit senses a state wherein the amount of current supplied to the semiconductor apparatus exceeds a predetermined amount, i.e., an overcurrent, an operation of the semiconductor apparatus is controlled and interrupted.
When the protection circuit is externally connected to the power semiconductor apparatus as described above, however, a system structure is undesirably large-sized and it is difficult to realize high reliability.
In consideration of the above problems, a composite function element wherein the power semiconductor apparatus and a circuit having an overcurrent protection function are arranged on a single chip has been proposed. This element is known as a DMOS (double diffused MOS) FET. More specifically, in the DMOSFET, an active element for controlling a power and control elements having sensing functions of, e.g., a temperature, a voltage, and a current, and a protection function utilizing these sensing functions are arranged on a single chip.
Such a power semiconductor apparatus includes a semiconductor element having an active function for controlling a load current In a junction of the semiconductor element having the active function, a load current flows and hence heat is generated. Therefore, for example, when a load is short-circuited and an over-current flows, an excessive current is supplied to the junction of the semiconductor element, and the temperature of the junction is abnormally increased. The semiconductor element may be damaged. For this reason, a protection operation is required to sense an abnormal increase in temperature of a semiconductor substrate on which the above-mentioned active elements are formed, and to ON/OFF-control the semiconductor element in response to the sensed output. A control element unit for performing the above protection operation and the semiconductor element for performing an active operation are formed on a single chip.
Most of the elements for constituting the above composite element have an extremely complicated element structure obtained by, e.g., p-n junction isolation or dielectric isolation. In these constituting elements, a parasitic operation tends to occur. Therefore, an effect of the composite element including the control elements on a single chip cannot be sufficiently enhanced. In addition, since the manufacturing process is complicated, cost of the apparatus is inevitably increased.
For example, a semiconductor circuit shown in FIG. 5 is considered to simplify the manufacturing process. In this circuit, a pair of MOS transistors P1 and P2 constitute a current mirror circuit. The MOS transistors P1 and P2 are controlled in response to an input signal from a gate input terminal, so that a load (not shown) connected to the drain terminals of the transistors is controlled.
For example, an enhancement type p-channel MOS transistor P3 is controlled by a voltage generated across a load current sensing resistor Rs connected to the current mirror circuit, and the load control MOS transistors P1 and P2 are controlled by the MOS transistor P3, thus protecting the circuit from an over-current.
In such a semiconductor circuit, the MOS transistor P3 senses a voltage across the resistor Rs. The voltage corresponding to a threshold voltage V.sub.T (about 1 V) of the MOS transistor P3 must be generated across the resistor Rs. For this reason, a current ratio of the MOS transistors P1 and P2 constituting the current mirror circuit is largely shifted from a ratio of the sizes of the MOS transistors, and power source voltage characteristics determine the current ratio. The temperature characteristic of the threshold voltage V.sub.T is about 3 mV/.degree. C., and the temperature characteristic of the current sensing is necessarily and largely increased.